1. Field of the Invention
The present invention relates to an antimony trioxide composition coated with hydrous zirconium oxide-silica and to a process of producing the composition.
2. Description of the Prior Art
At present, plastics are used in a number of diverse fields such as building materials, electric parts, automobiles, ships, airplanes, etc., due to their excellent physical and chemical properties, and the demand therefor is constantly increasing. On the other hand, from the view point of preventing fires and to protect human life, it has recently been required in many fields, especially those listed above, to impart flame retardance to plastics. Thus, numerous attempts to render combustible materials flame retardant or further incombustible have been made, and various flame retardants and the assistants therefor have been developed. Typical examples of such flame retardant and assistants are organic phosphorus compounds and antimony trioxide.
Antimony trioxide itself is inactive and shows almost no flame retarding effect by itself, but since it shows a flame retarding effect upon interaction thereof with other materials used therewith, antimony trioxide is considered a flame retardant assistant.
The various antimony chlorides formed by the following reactions of antimony trioxide under heating in the presence of halogen compounds show a flame preventing action: ##STR1##
SbCl.sub.3 and SbBr.sub.3 are volatile and reactive. The antimony halides promote the transfer of halogen and the formation of carbide in the solid phase and act as a free radical acceptor in the vapor state, which provides a flame prevention effect. The action and mechanism of inorganic materials such as antimony trioxide which are used as a flame retarding assistant together with halide-type flame retardants have not yet been completely identified, but one of the important actions thereof is believed to be that it has the effect of diluting plastic organic materials, and further it has no influence on the flame retardant in the normal state, but, when the flame retardant is heated together with a polymer at the outset of combustion, it acts to properly reduce the decomposition point of the flame retardant to accelerate the flame retarding effect. As a matter of course, it covers the surface of the polymer at combustion to provide a covering effect.
Since antimony trioxide exhibits an excellent effect as a flame retarding assistant, it has long been used in large quantities. However, since it has the serious faults that when it is used in a plastic for molding, or for the adhesion of plastics or in rubbers, it is converted into antimony trichloride to cause yellowing or into antimony trisulfide which causes a black-brown discoloring by chlorine gas, sulfides, and other gaseous atmospheres used for vulcanization; thus, the usable range of antimony trioxide is, as a matter of fact, greatly restricted. To overcome these faults, it has generally been proposed to improve the chemical resistance of antimony trioxide by diluting the same with other inorganic materials.
The inventors attempted to coat antimony trioxide with silica to improve the chemical resistance thereof and found that such coated antimony trioxide could be used in cases which did not require high dispersion. However, in such a coating the bonding between the surface of an antimony trioxide particle and the coating was insufficient. Thus, when strong mechanical shear was applied to the antimony trioxide particles by a dispersing means to disperse the particles in a medium as a flame retardant or by hot rolls or a molding machine, the silica coating was damaged or stripped from the surface of the particles to greatly reduce the covering effect.
Considering the aforesaid problems, the inventors performed research to improve the most serious fault, i.e., the poor chemical resistance of antimony trioxide as is currently largely used in these fields, and, as a result, the inventors succeeded in remarkably improving the chemical resistance of antimony trioxide used for these purposes.